The invention relates generally to charging of rechargeable energy storage systems (e.g., batteries and electric double-layer capacitors and the like) and more particularly to conductive charging systems that counter a possibility of triggering a residual-current device (RCD) when an external charging power source is coupled to an internal, isolated, energy storage system. In the United States, RCDs are referred to as a ground fault circuit interrupter (GFCI), ground fault interrupter (GFI), an appliance leakage current interrupter (ALCI), or the like.
Energy storage systems used in electric vehicles (EVs) and other industrial applications store significant amounts of energy that is dangerous when improperly handled. Many safety features are adapted and incorporated into these applications to enhance safety. One of those features includes use of an isolated ground for the energy storage system. A person is able to touch both a storage element of the energy storage system and a chassis of the EV without being shocked.
In the case of rechargeable storage elements in the energy storage system, it is common to use an “external” charging system or external source of charging energy (herein, external charging system includes external sources of charging energy) coupled to the energy storage system. For EVs, it is desirable that these charging systems be available at locations that are convenient for the users and operators of those vehicles. Among these locations, the residence of the user is often a prime location for installation of a charging system. Charging systems for EVs, because they provide high energy, are sometimes special installations with particular design considerations. More commonly, it is desirable to provide for the user to simply plug their EV into an AC line voltage plug at their residence to initiate charging. The charging often requires more than an insignificant amount of time, thus the user will “plug” their vehicle into the wall socket and then leave the charging location to occupy their time until the vehicle is charged and they have desire to use the vehicle. It is common for electrical systems of residences to employ an RCD in general, and particularly at the charging location.
This charging system has a ground as well. Because the (+) and (−) of the energy storage system is isolated, the actual voltage level of the (+) and (−) of the energy storage system could be different, and in fact is likely different from, the voltage of the AC phases of the charging system. At the moment that the charging system is coupled to the energy storage system, the relative voltages are equalized to the same voltage. It is this equalization that can result in a current flow that the RCD may detect as an unsafe residual current and disable current flow from the charging station into the energy storage system.
This is inconvenient for the user. The user had left the location of the charging station and is likely unaware that the charging had been suspended. It is the case that returning to the EV after the allotted time with an anticipation of use of the vehicle only to find the state of charge of the energy storage system unchanged may engender a negative reaction in the user. The user must make alternate arrangements to accommodate the situation that the EV is not ready as planned, which can have varying consequences of its own depending upon the nature and availability of alternatives to the EV.
What is needed is a charging apparatus and method that accommodates potential residual current when electrical grounds of the charger and the energy storage system are equalized at the moment of initiating charging.